An $(r,w;d)$ cover-free family $(CFF)$ is a family of subsets of a finite set such that the intersection of any $r$ members of the family contains at least $d$ elements that are not in the union of any other $w$ members. The minimum number of elements for which there exists an $(r,w;d)-CFF$ with $t$ blocks is denoted by $N((r,w;d),t)$. 
In this paper, we show that the value of $N((r,w;d),t)$ is equal to the $d$-biclique covering number of the bipartite graph $I_t(r,w)$ whose vertices are all $w$- and $r$-subsets of a $t$-element set, where a $w$-subset is adjacent to an $r$-subset if their intersection is empty. Next, we introduce some new bounds for $N((r,w;d),t)$. For instance, we show that for $r\geq w$ and $r\geq 2$ 
$$ N((r,w;1),t) \geq c{{r+w\choose w+1}+{r+w-1 \choose w+1}+ 3 {r+w-4 \choose w-2} \over \log r} \log (t-w+1),$$ where $c$ is a constant satisfies the well-known bound $N((r,1;1),t)\geq c\frac{r^2}{\log r}\log t$. Also, we determine the exact value of $N((r,w;d),t)$ for some values of $d$. Finally, we show that $N((1,1;d),4d-1)=4d-1$ whenever there exists a Hadamard matrix of order 4d.

Some New Bounds For Cover-Free Families Through Biclique Cover

Some new bounds for cover-free families through biclique covers

A (w,r) cover-free family is a family of subsets of a finite set such that no intersection of w members of the family is covered by a union of r others. A (w,r) superimposed code is the incidence matrix of such a family. Such a family also arises in cryptography as the concept of key distribution pattern. In the present paper, we give some new results on superimposed codes. First we construct superimposed codes from super-simple designs which give us results better than superimposed codes constructed by other known methods. Next we prove the uniqueness of the (1,2) superimposed code of size 9 × 12, the (2,2) superimposed code of size 14 × 8, and the (2,3) superimposed code of size 30 × 10. Finally, we improve numerical values of upper bounds for the asymptotic rate of some (w,r) superimposed codes. © 2004 Wiley Periodicals, Inc.

Some new results on superimposed codes

For a binary code Γ of length v, a v-word w produces by a set of codewords {w1,...,wr}⊆Γ if for all i=1,...,v, we have wi∈{w1i,...,wri} . We call a code r-secure frameproof of size t if |Γ|=t and for any v-word that is produced by two sets C1 and C2 of size at most r then the intersection of these sets is nonempty. A d-biclique cover of size v of a graph G is a collection of v-complete bipartite subgraphs of G such that each edge of G belongs to at least d of these complete bipartite subgraphs. In this paper, we show that for t≥2r, an r-secure frameproof code of size t and length v exists if and only if there exists a 1-biclique cover of size v for the Kneser graph KG(t,r) whose vertices are all r-subsets of a t-element set and two r-subsets are adjacent if their intersection is empty. Then we investigate some connection between the minimum size of d-biclique covers of Kneser graphs and cover-free families, where an (r,w;d) cover-free family is a family of subsets of a finite set such that the intersection of any r members of the family contains at least d elements that are not in the union of any other w members. Also, we present an upper bound for 1-biclique covering number of Kneser graphs.

https://hal.inria.fr/hal-00990601/document

Secure frameproof codes through biclique covers

The study of separating systems was apparently originated in the problems of critical race-free coding for states of discrete automata. These problems are connected with the so-called races of memory cells of automata, i.e., with the fact that, during the transition from one stable inner state, s1, to another, s2, the binary memory cells of an asynchronous automaton do not change their states simultaneously. This transition will be denoted as s1 → s2. The collection of states of memory cells of the automaton at a given time instant forms its inner state. A consequence of this time nonuniformity in the course of the transition from one state to another is the rise of intermediate states s (a)

/pdf/separating-systems-and-new-scopes-of-its-application-zuaj8qgh7g.pdf

Separating Systems and New Scopes of Its Application

A (w, r)-cover-free code is the incidence matrix of a family of sets where no intersection of w members of the family is covered by the union of r others. We obtain a new condition in view of which (w, r)-cover-free codes with a simple structure are optimal. We also introduce (w, r)-cover-free codes with a constraint set.

On the Optimality of Trivial ( w , r )-Cover-Free Codes

Four new results on the uniqueness of optimal superimposed codes are presented, namely, the uniqueness of (w, r) superimposed codes of size N × T with $$N = \left( {\begin{array}{*{20}c} {w + r + 1} \\ w \\ \end{array} } \right)$$ and T = w + r + 1 and the uniqueness of (2, 2) superimposed codes of size 18 × 9, (2, 2) superimposed codes of size 14 × 7, and (3, 3) superimposed codes of size 66 × 11.

Uniqueness of some optimal superimposed codes

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On the Optimality of Trivial ( w , r )-Cover-Free Codes

Uniqueness of some optimal superimposed codes